1. Field of the Invention
This invention relates to a navigational system for determining the bearing and distance of one aircraft or spacecraft with respect to another, the two objects being above ground in the Earth's atmosphere or in outer space. It more particularly relates to a radio navigation system using a set of pulses of unmodulated radio beacons of a specific frequency transmitted from a phased array antenna.
2. Description of the Prior Art
U.S. Pat. No. 4,823,137 to Mallick describes a navigational system using an array of fixed, directional antennas for broadcasting unmodulated radio signals at several different frequencies. Each antenna is conductively coupled to a transmitter that provides the unmodulated radio signal to the antenna, and each antenna transmits a radio signal of a different frequency from the frequencies of transmission of other antennas in the array. Since each antenna is pointing in a different direction from other antennas in the array, each antenna's radio signal radiation pattern will vary with angle. The bearing of the antenna array is calculated by using an array of receivers, where each receiver is tuned to a frequency identical to the frequency of each transmitting antenna. Each receiver receiving the unmodulated radio signal for each frequency measures the received signal power for each frequency and calculates the received power ratios for all signals. Calculated power ratios are then compared to a library of power ratios and their corresponding angles with respect to the transmitting array to determine the bearing of the receiver with respect to the transmitting array. Range is calculated by comparing the received power of the radio signals with the known transmitted power. Using the equation defining the inverse power law can determine the distance between the transmitting array and the receivers. Another way distance is determined is by having the receiver array send out a query pulse that triggers the antenna array to transmit their signals and measuring the time that the signals are received from the time that the query pulse is transmitted. Dividing the elapsed time by two and multiplying that time with the speed of light will determine the distance between the transmitting antennas in the receiver array.
U.S. Pat. No. 5,915,278 to Mallick describes a navigational system using a flat-panel array of microstrip antenna elements to transmit the unmodulated radio signals instead of using a set of directional antennas. Each element on the flat-panel array is conductively coupled to a transmitter that causes the microstrip antenna to transmit an unmodulated radio signal having a different frequency and a different radiation pattern from the other microstrip antennas in the array. The different radiation patterns can be created by using different antenna geometries, putting dielectric lens over some antennas, or by introducing a slight tilt of the antennas on the panel with respect to the other antennas on the array. Since each antenna produces a different radiation pattern with respect to any other antennas in the array, each radio signal received by a receiver array some distance from the transmitting array has a different signal power. Bearing can be calculated by calculating the ratios of each signal power of each frequency and comparing them to a library of power ratios as a function of frequency. The corresponding angle to each power ratio in the library will determine the bearing. Range is calculated by methods similar to that in U.S. Pat. No. 4,823,137.
U.S. Pat. Nos. 4,823,137 and 5,915,278 to Mallick describe navigational system utilizing several radio frequencies which require a plurality of radio signal transmitters and an array of receivers. However, in many aircraft and spacecraft there are size and weight limitations that require that a simple and more compact electronic system be used. A system that can determine bearing and range from a single radio frequency could conform to the aforementioned size and weight limitations.
Other known existing radio navigation devices transmit either a rotating and/or modulating radio beacon. Rotating the radio beacon is disadvantageous because the navigational signal is transmitted to only a small volume of space at any one time. Modulating the radio beacon is disadvantageous because the time consumed in modulating and demodulating the signal limits the repeat transmission rate of the navigational signal. Systems that utilize unmodulated radio beacons of different frequencies require several radio sources of different frequencies and several receivers to receive the unmodulated signals. This is disadvantageous because it requires more equipment than utilizing just a single frequency.
U.S. Pat. No. 4,017,860 to Earp teaches using a fixed planar matrix of antennas with programmed excitation to produce a moving pencil beam of radiation. This produces an artificial Doppler signal component as observed at a remote receiving station. Navigational information is derived from the signal by using the principle that the Doppler shift in frequency is proportional to the cosine of the angle which a radio receiver of the system subtends with respect to the line of movement of the source.